346 WILLIS : MORPHOLOGY OP THE PODOSTBMACEÆ 
one flower instead of many. The axis, hitherto evanescent, 
comes above the surface of the thallus, and developes more 
leaves. These differ from the previous vegetative leaves, in 
that the sheathing base is larger and the blade smaller. The 
axis rarely exceeds a few millimetres in length, exclusive 
of the flower-stalk. The tips of the leaves shrivel away, 
and very soon only the sheathing lower parts are left. 
There are usually about four of these “ bracts ” below the 
flower, the two upper being larger than the two lower ; 
their shape is usually long and narrow, but sometimes shows 
an approximation to the cowl or helmet form characteristic 
of D. stylosa, &c. The upper side of the bract is thicker 
than the lower, just as in the unaltered leaves of Podoste- 
mon subulatus. At the end of the axis is the solitary flower, 
enclosed as yet in its spathe, the latter being very like that 
of Podostemon. PI. XYIII., flg. 11, shows a section at this 
period through the thallus and secondary shoot, and shows 
the shrivelled cortex and the vascular bundle branching to 
the secondary shoot. 
The spathe remains closed until exposure to the air, with 
the flower inside it quite ready for expansion. Presently, 
with the stiffening of the tissues the flowering part of the 
thallus becomes fairly rigid, only the non-floriferous tip still 
undulating in the water ; very often the flowering part 
becomes more or less erect upon the rock. Soon after, with 
the continual fall of the water-level, the flower-spathes 
begin to be exposed to the air, and as this happens they split 
and the flowers emerge. Thus, as all the flowers are ready 
for immediate expansion, it happens that the actual order 
in which they ultimately open is determined, not by any 
morphological construction of the plant itself, but by the 
orientation of the floriferous thallus with regard to the 
water-level. As very often the thallus is nearly erect, it 
happens that the order of opening is a descending one, so 
commonly in fact that this plant is often quoted in the text 
books as one in which an apparent raceme is really centri- 
fugal. If the thallus happen to lie out horizontally, and be 
